Transmembrane 4 L6 family member 20 (TM4SF20) is a four-transmembrane protein expressed on a cell membrane and belongs to the L6 tetraspanin family. TM4SF1 (L6), TM4SF4 (ILTMP), TM4SF5, and the like are known as members of the L6 tetraspanin family. All of them are 4-transmembrane proteins which have a short intracellular region at the N and C termini and have a short extracellular region between a transmembrane region 1 (TM1) and TM2 and a long extracellular region between TM3 and TM4 (Non Patent Literature 1). TM4SF1 is expressed in lung cancer, colon cancer, breast cancer, and ovarian cancer, and the clinical tests of anticancer agents have been practiced using an anti-TM4SF1 antibody (Non Patent Literature 2). Moreover, the anti-TM4SF1 antibody is known to reduce the invasive capacity of lung cancer cells (Non Patent Literature 3). TM4SF4 has been reported to be expressed on the luminal side of intestinal epithelial cells (Non Patent Literature 4). TM4SF5 has been found as a molecule highly expressed in pancreatic cancer (Non Patent Literature 5).
While the L6 tetraspanin family belongs to the tetraspanin superfamily, many members of the tetraspanin superfamily are known to bind to integrin (Non Patent Literatures 6 and 7). TM4SF5 is also known to bind to integrin α2 and further induce EMT (epithelial-mesenchymal transition) by focal adhesion kinase-mediated signal transduction (Non Patent Literatures 8 and 9).
For TM4SF20, its enhanced expression is known in Barrett's esophagus, in the swine small intestine infected with Salmonella, and in a human lung cancer cell line A549 expressing severe acute respiratory syndrome (SARS) virus proteins (Non Patent Literatures 10 to 12). According to reports, its expression is reduced in stomach cancer among cancers (Patent Literature 1). On the other hand, a patent application has reported use of TM4SF20 as an insulin secretion inducer (Patent Literature 2). However, there are few reports as to TM4SF20, and its physiological functions or expression specific for cancer tissues have not been reported so far.